This invention relates to the preparation of a new catalyst which is particularly useful in hydrocarbon reforming processes. More particularly, this invention relates to a new rhenium-promoted catalyst which does not require presulfiding treatment in hydrocarbon reforming service.
In the petroleum refining industry, catalytic reforming is a widely employed commercial process for upgrading the antiknock quality of naphtha as a blending stock for motor fuels, particularly gasoline. Typically, the most desirable reaction taking place during reforming is the dehydrogenation of naphthenes to form aromatics and hydrogen, although a number of other reactions take place including cyclization of normal paraffins to naphthenes, isomerization of normal paraffins to isoparaffins, and hydrocracking which can be excessive and in severe conditions termed "hydrogenolysis" produces unwanted light gases, decreasing yield of gasoline-boiling products.
Catalysts containing platinum group metals have been conventionally employed in reforming processes in order to promote dehydrogenation reactions which are also favored by operation at lower pressures and high severity. However, operation under such conditions has generally caused deactivation of the typical platinum catalyst due to coke formation on the catalyst requiring frequent catalyst regeneration. In U.S. Pat. No. 3,415,737 (1968), which is incorporated herein by reference, Kluksdahl discloses that alumina supported reforming catalysts comprising a rhenium component as a promoter for typical platinum catalyst provide an improved activity maintenance permitting operation at lower pressure and higher severity to allow improved selectivity for gasoline yields without shortening regeneration cycle times. However, this patent points out that a disadvantage of the disclosed rhenium-promoted catalyst is the necessity to subject the catalyst to presulfiding treatment in order to prevent excessive hydrocracking in the reforming process which has been a commercial disadvantage with the employment of such rhenium-promoted catalyst compounded by the fact that it is generally necessary to resulfide such catalyst after each regeneration. Furthermore, if such presulfiding treatment results in the catalyst becoming oversulfided, the reforming activity of the catalyst is poisoned. Thus, only when the correct sulfur level is achieved in the presulfiding treatment have the benefits of using rhenium-promoted platinum reforming catalyst been obtainable. Typical commercial presulfiding treatment is carried out by contacting the bed of catalyst with a mixture of hydrogen sulfide and a gas such as hydrogen or nitrogen.
Antos et al. in U.S. Pat. No. 4,124,491 (1978) disclose a reforming process employing a selectively sulfided acidic multi metallic catalytic composite comprising a combination of catalytically effective amounts of a platinum group component, a sulfided rhenium component and a halogen component with a porous carrier material formed from Ziegler alumina; the patent does not suggest that the catalyst contain a silica component. Furthermore, as pointed out in this patent in column 12, beginning at line 43, the disclosed catalyst requires a selective sulfiding treatment in order to convert substantially all of the rhenium component to a sulfided state.
Collins et al. in U.S. Pat. No. 4,124,490 (1978) disclose a two-stage reforming process employing a catalyst comprising a major amount of a porous solid support, preferably alumina, at least one platinum-group metal component, and at least one rhenium component; in addition to alumina, the solid porous support can contain minor proportions of other well-known refractory inorganic oxides such as silica, zirconia, magnesia, and the like, however, "the most preferred support is substantially pure alumina derived from hydrous alumina predominating in alumina monohydrate." Collins et al. do not disclose an example of a catalyst containing silica, nor do they discuss the presulfiding treatment of the catalyst to prevent excessive hydrocracking in the reforming process.
Wilson et al. in U.S. Pat. No. 3,986,947 (1976) disclose a reforming process employing a catalyst composed of a noble metal, particularly platinum, deposited or impregnated on silica which is dispersed in an alumina matrix; while Wilson et al. suggest that rhenium can also be deposited on the silica no example of a catalyst containing rhenium is presented nor is consideration given to consequent presulfiding treatment. In addition, Wilson et al. do not suggest that the alumina matrix can be impregnated with the noble metal. Moreover, Wilson et al. limit the amount of alumina in their catalyst to about 5-30 wt. %, preferably not more than 15 wt. % of the catalyst.
We have now developed a new rhenium-promoted catalyst which eliminates the need to presulfide the catalyst in hydrocarbon reforming operations, which has been a necessary pretreatment of conventional rhenium-promoted catalyst in order to prevent their excessive hydrocracking activity. The catalyst of this invention comprises a rhenium component which is supported on silica and a platinum-group component deposited on both the silica and on an alumina component in composite which alumina is preferably the major component of the catalyst on weight basis. This catalyst appears to allow the interaction of the silica support for the rhenium and a portion of the platinum-group metal component to facilitate reduction of the rhenium under reforming conditions wherein the reduced rhenium component does not exhibit the excessive hydrocracking activity experienced with unsulfided conventional rhenium-promoted reforming catalyst; in addition, the catalyst exhibits the superior reforming activity afforded by a portion of the platinum-group metal supported on the alumina component.
The general object of this invention is to provide a rhenium-promoted catalyst which can be employed in hydrocarbon reforming processes without need for the conventional presulfiding treatment required by the prior art rhenium-promoted catalysts. Another object is to eliminate such presulfiding treatment without sacrifice in the reforming performance of the catalyst.
The objects of this invention can be attained with a catalyst comprising at least one platinum-group metal deposited on a composite comprising (1) alumina and (2) rhenium deposited on silica.
Briefly the catalyst of this invention can be prepared by a method comprising:
(1) forming a composition comprising rhenium deposited on silica;
(2) drying the composition of step (1);
(3) forming a composition comprising the product of step 2 and an alumina component;
(4) impregnating the composition of step 3 with a solution comprising platinum-group metal; and
(5) calcining the product of step 4.